Oxidation and Ablation Behavior of Particle-Filled SiCN Precursor Coatings for Thin-Film Sensors

Polymer-derived ceramic (PDC) thin-film sensors have a very high potential for extreme environments. However, the erosion caused by high-temperature airflow at the hot-end poses a significant challenge to the stability of PDC thin-film sensors. Here, we fabricate a thin-film coating by PDC/TiB2/B composite ceramic material, which can be used to enhance the oxidation resistance and ablation resistance of the sensors. Due to the formation of a dense oxide layer on the surface of the thin-film coating in a high-temperature air environment, it effectively prevents the ingress of oxygen as a pivotal barrier. The coating exhibits an exceptionally thin oxide layer thickness of merely 8 μm, while its oxidation resistance was rigorously assessed under air exposure at 800 °C, proving its enduring protection for a minimum duration of 10 h. Additionally, during ablation testing using a flame gun that can generate temperatures of up to 1000 °C, the linear ablation rate of thin-film coating is merely 1.04 μm/min. Our analysis reveals that the volatilization of B2O3 occurs while new SiO2 is formed on the thin-film coating surface. This phenomenon leads to the absorption of heat, thereby enhancing the ablative resistance performance of the thin-film sensor. The results indicate that the thin-film sensor exhibits exceptional resistance to oxidation and ablation when protected by the coating, which has great potential for aerospace applications.


Introduction
The growing demand for monitoring operations in harsh environments has stimulated the advancement of high-temperature sensors. High-temperature thin-film sensors offer significant potential for integration into high-temperature components owing to their advantageous attributes, including micrometer-scale thickness, negligible mass, nonintrusive nature, and minimal interference with surface airflow and component vibration modes [1][2][3]. Compared with metal materials, the precursors of PDC are generally liquid polymers, and the liquid precursors forming-curing-crosslinking-pyrolysis to form a ceramic structure. Its working ability in extreme environments, such as oxidation resistance and thermal shock resistance, is significantly better than alloys. Polymer-derived ceramics (PDCs), due to their excellent resistance to oxidation and high temperature properties [4], are widely used in high-temperature bulk [5][6][7] and thin-film sensors. PDC thin-film sensors [8] can be fabricated by an in situ direct writing method, which has a great advantage in the high-temperature sensor field.
Currently, a significant amount of research is focused on exploring the performance of PDC thin-film sensors, including strain gauges [8,9], temperature sensors [10][11][12], and heat flux sensors [13]. Cui [14] developed a temperature-sensitive sensor using a SiCN film that was less than 100 µm thickness, with a maximum measurement temperature of 800 • C. Wu et al. [9] fabricated TiB 2 /SiCN thin-film strain gages, which can work close onto the alumina substrate. After the direct writing is complete, it is placed on a heated platform and cured at 100 °C for 10 min. Finally, the film is annealed in the air at 800 °C in the muffle furnace and cooled with the furnace; it reaches the indoor temperature, and can be taken out. The thin-film coating is thus obtained.
Currently, we are applying the prepared thin-film above the thin-film sensor. By coating the thin-film on the alumina substrate where the sensitive gate is fabricated, Please delete it. The correct statement should be: we can obtain a double-layer thin-film sensor [14]. The resistance of this sensor is characterized by cycling stability at room temperature to 800 °C [14].

Protection Performance Tests of the Thin-Film Coatings
The furnace was set to 800 °C to test the oxidation resistance of the coating. A thermogravimetric analyzer (TG-DSC, TGA/DSC, STA449F5) was used for the analysis of TiB2, B and thin-films. At 800 °C, the oxidation characteristic time was heated at a rate of 10 K/min. The following equation calculated cumulative mass change percentages (∆mass%) of the powders:

∆mass% = −
where and are the mass of the powder before and after oxidation for t minutes. The curves of mass change with oxidation time were given according to the above calculation formula.
To assess the ablation resistance of the film, a simplified film ablation system was established ( Figure 2). The system utilized a flame gun powered by a 95% butane gas stream, with a test temperature of 1000 °C. The flame gun nozzle had an inner diameter of 20 mm and was positioned 6 cm away from the sample. The butane gas flow rate was set at 0.03 L/s. A thermocouple was centrally placed on the sample to monitor the realtime temperature. The film underwent continuous ablation in the flame for durations of 4 min, 10 min, 15 min, and 30 min, and the linear ablation rate was subsequently calculated. The line ablation rate was used to evaluate the ablation resistance.

= ∆
where is the line ablation rate, ∆m is the coating thickness change, and t is the ablation time. Currently, we are applying the prepared thin-film above the thin-film sensor. By coating the thin-film on the alumina substrate where the sensitive gate is fabricated, Please delete it. The correct statement should be: we can obtain a double-layer thin-film sensor [14]. The resistance of this sensor is characterized by cycling stability at room temperature to 800 • C [14].

Protection Performance Tests of the Thin-Film Coatings
The furnace was set to 800 • C to test the oxidation resistance of the coating. A thermogravimetric analyzer (TG-DSC, TGA/DSC, STA449F5) was used for the analysis of TiB 2 , B and thin-films. At 800 • C, the oxidation characteristic time was heated at a rate of 10 K/min. The following equation calculated cumulative mass change percentages (∆mass%) of the powders: where m 0 and m t are the mass of the powder before and after oxidation for t minutes. The curves of mass change with oxidation time were given according to the above calculation formula.
To assess the ablation resistance of the film, a simplified film ablation system was established ( Figure 2). The system utilized a flame gun powered by a 95% butane gas stream, with a test temperature of 1000 • C. The flame gun nozzle had an inner diameter of 20 mm and was positioned 6 cm away from the sample. The butane gas flow rate was set at 0.03 L/s. A thermocouple was centrally placed on the sample to monitor the real-time temperature. The film underwent continuous ablation in the flame for durations of 4 min, 10 min, 15 min, and 30 min, and the linear ablation rate was subsequently calculated. The line ablation rate was used to evaluate the ablation resistance.
where R m is the line ablation rate, ∆m is the coating thickness change, and t is the ablation time.

Characterization
The sensors were primarily characterized using a profilometer (Dektak XT) to measure their thickness, and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) using a JSM-IT500A instrument from JEOL in Tokyo, Japan, to analyze their morphology and elemental composition. X-ray diffraction (XRD) analysis was performed

Characterization
The sensors were primarily characterized using a profilometer (Dektak XT) to measure their thickness, and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) using a JSM-IT500A instrument from JEOL in Tokyo, Japan, to analyze their morphology and elemental composition. X-ray diffraction (XRD) analysis was performed using a Shimadzu XRD-6100 instrument. The output resistance of the thin-film sensors was measured using a Keysight 34972A data acquisition device (DAQ). Figure 3a shows the XRD pattern of the prefabricated coating; the TiB2 phases are detected. Figure 3b,c depict scanning electron microscope (SEM) images of the prefabricated coating surface, providing insight into the distribution and morphology of the raw material particles. The surface exhibits a relatively rough texture and contains noticeable particulate features. EDS analysis of spot 1 (Figure 3c) reveals the presence of titanium (Ti) and boron (B) on the surface. Furthermore, Figure 3(c1) exhibits a uniform distribution of oxygen (O) and silicon (Si) elements, indicating the effective bonding role of PSN2 during the coating fabrication process. Consequently, the surface particles primarily consist of TiB2 and B. Figure 3d displays a cross-sectional view of the prefabricated coating, showcasing a uniform thickness of approximately 30 µm applied to the alumina surface. The Energy Dispersive Spectrometer (EDS) analysis (Figure 3(d1)) clearly distinguishes titanium (Ti) elements and reveals relatively even distribution of silicon (Si) and aluminum (Al) elements, where Al is the element of the base (Al2O3). Notably, no instances of delamination or peeling are observed, except for a few larger TiB2 particles measuring 1 µm. The coating is successfully obtained at room temperature and exhibits excellent adhesion to the alumina substrate.  Figure 3a shows the XRD pattern of the prefabricated coating; the TiB 2 phases are detected. Figure 3b,c depict scanning electron microscope (SEM) images of the prefabricated coating surface, providing insight into the distribution and morphology of the raw material particles. The surface exhibits a relatively rough texture and contains noticeable particulate features. EDS analysis of spot 1 (Figure 3c) reveals the presence of titanium (Ti) and boron (B) on the surface. Furthermore, Figure 3(c1) exhibits a uniform distribution of oxygen (O) and silicon (Si) elements, indicating the effective bonding role of PSN2 during the coating fabrication process. Consequently, the surface particles primarily consist of TiB 2 and B. Figure 3d displays a cross-sectional view of the prefabricated coating, showcasing a uniform thickness of approximately 30 µm applied to the alumina surface. The Energy Dispersive Spectrometer (EDS) analysis ( Figure 3(d1)) clearly distinguishes titanium (Ti) elements and reveals relatively even distribution of silicon (Si) and aluminum (Al) elements, where Al is the element of the base (Al 2 O 3 ). Notably, no instances of delamination or peeling are observed, except for a few larger TiB 2 particles measuring 1 µm. The coating is successfully obtained at room temperature and exhibits excellent adhesion to the alumina substrate.

Oxidation Behavior of the Thin-Film Coatings
To evaluate the oxidation resistance of the coating, we obtained an isothermal curve by subjecting the coated material to a temperature of 800 • C for 12h and air flow of 50 mL/min ( Figure 4). The curve exhibits three distinct stages: a rapid mass increase (Stage I), a slower mass increase (Stage II), and a plateau (Stage III). After 80 min of oxidation of the coating, its mass increased sharply, reaching 0.15 wt%. According to the TG-DSC and isothermal oxidation curves (Figure 5a,b) of B powder and TiB 2 powder, the furnace is heated up to 800 • C with a rate of 10 • C/min, and keeping for 50 mL/min of the air flow. It can be seen that there is a large amount of oxygen diffusion in the powder, oxidizing TiB2 and B particles, producing TiO 2 and B 2 O 3 (Figure 5c,d). Based on the literature of the antioxidant thin-film coating, it is required to have a good coefficient of thermal expansion with the substrate and sensitive layers [25,26]. The average alumina substrate thermal expansion coefficient [27] is measured at 7.1 × 10 −6 /K −1 . Additionally, the average thermal expansion coefficients of B and TiB 2 are 6.4 × 10 −6 /K −1 and 7.0 × 10 −6 /K −1 , respectively [28,29]. Notably, TiB 2 is a well-established high-temperature material that has been successfully utilized in high-temperature thin-film applications [9,30]. Consequently, the combination of B/TiB 2 offers significant advantages over using only B as the filler, as it achieves a superior thermal match with both the substrate and the sensitive layer of the thin-film sensor. This enhanced thermal match ensures improved overall performance and stability for the thin-film sensor. As oxidation time increases, mass gain enters a slow phase (II stage). The mass plateau stage is in III stage with a mass gain of 0.2 wt%.

Oxidation Behavior of the Thin-Film Coatings
To evaluate the oxidation resistance of the coating, we obtained an isothermal curve by subjecting the coated material to a temperature of 800 °C for 12h and air flow of 50 mL/min ( Figure 4). The curve exhibits three distinct stages: a rapid mass increase (Stage I), a slower mass increase (Stage II), and a plateau (Stage III). After 80 min of oxidation of the coating, its mass increased sharply, reaching 0.15 wt%. According to the TG-DSC and isothermal oxidation curves (Figure 5a,b) of B powder and TiB2 powder, the furnace is heated up to 800 °C with a rate of 10 °C/min, and keeping for 50 mL/min of the air flow. It can be seen that there is a large amount of oxygen diffusion in the powder, oxidizing TiB2 and B particles, producing TiO2 and B2O3 (Figure 5c,d). Based on the literature of the antioxidant thin-film coating, it is required to have a good coefficient of thermal expansion with the substrate and sensitive layers [25,26]. The average alumina substrate thermal expansion coefficient [27] is measured at 7.1 × 10 −6 /K −1 . Additionally, the average thermal expansion coefficients of B and TiB2 are 6.4 × 10 −6 /K −1 and 7.0 × 10 −6 /K −1 , respectively [28,29]. Notably, TiB2 is a well-established high-temperature material that has been successfully utilized in high-temperature thin-film applications [9,30]. Consequently, the combination of B/TiB2 offers significant advantages over using only B as the filler, as it achieves a superior thermal match with both the substrate and the sensitive layer of the thin-film sensor. This enhanced thermal match ensures improved overall performance and stability for the thin-film sensor. As oxidation time increases, mass gain enters a slow phase (II stage). The mass plateau stage is in III stage with a mass gain of 0.2 wt%.
It is also found that the initial oxidation temperature of TiB2 powder in Figure 5a is about 450 °C [31], which is significantly lower than that of B powder at 600 °C [32]. At the same time, the oxidation trend of TiB2 in air is significantly greater than that of B. Therefore, the presence of B allows the coating to better form an oxide layer. At the same time, SiCN undergoes a high-temperature reaction, leading to the generation of SiO2, which effectively repairs any cracks caused by oxidation. And the changes of substances before and after exposure to high temperatures of the thin-film coatings are analyzed using Xray photoelectron spectroscopy (XPS). The full spectra (Figure 6a) reveal a noticeable in-  The phase transformation of the coating during the oxidation process gated by XRD analysis for different oxidation times (Figure 7). After 1 h of oxi B2O3 and TiO2 phases appeared. It showed that a glassy oxide layer was fo    It is also found that the initial oxidation temperature of TiB 2 powder in Figure 5a is about 450 • C [31], which is significantly lower than that of B powder at 600 • C [32]. At the same time, the oxidation trend of TiB 2 in air is significantly greater than that of B. Therefore, the presence of B allows the coating to better form an oxide layer. At the same time, SiCN undergoes a high-temperature reaction, leading to the generation of SiO 2 , which effectively repairs any cracks caused by oxidation. And the changes of substances before and after exposure to high temperatures of the thin-film coatings are analyzed using X-ray photoelectron spectroscopy (XPS). The full spectra (Figure 6a)    The phase transformation of the coating during the oxidation process was investigated by XRD analysis for different oxidation times (Figure 7). After 1 h of oxidation, SiO 2 , B 2 O 3 and TiO 2 phases appeared. It showed that a glassy oxide layer was formed on the surface of the coating, and after 5 h, the peak strength of B 2 O 3 (Figure 7) increased, indicating that the density of the antioxidant layer was also enhanced (Figure 8b,c). Figure 8 shows the evolution of surface and cross-sectional topography during oxidation of the coating. After oxidation at 800 • C for 1 h (Figure 8a), 5 h (Figure 8b), 10 h (Figure 8c), an oxide film is formed on its surface, which is dense and has no micropores. According to the specific gravity analysis of the elements of points EDS (Figure 8(a3,b3,c3)), the O content is higher, followed by the B content. Combined with the results of XRD (Figure 7), a dense B 2 O 3 · SiO 2 glass layer forms on the surface of the coating. At the same time, the cross-sectional topography of the coating is analyzed. After 1 h of oxidation (Figure 8(a1,a2)), the O element on the outside of the coating is higher than the inside, and the distribution of Ti elements is granular, indicating that the particles inside the coating are not completely oxidized. After 10 h of oxidation (Figure 8(b1,b2)), a dense oxide layer (thickness:~8 µm) is formed on the surface of the coating, the surface of which is covered by a thick glass layer.

Ablation Behavior of the Coated Samples at Different Times
To evaluate the protective performance of antioxidant films against erosion in highvelocity airflow environments, a flame spray apparatus was employed to assess their ablation resistance. Figure 9a illustrates the film's ablation process at a central temperature of 1000 • C and the thickness change of the ablation pit is calculated. Figure 9b presents the linear ablation rates of the film samples at different time intervals, revealing rates of 1.42 µm/min, 1.41 µm/min, and 1.04 µm/min at 4 min, 10 min, and 15 min, respectively. These results indicate a progressive reduction in film thickness and ablation rate as the duration of ablation increases. At the same time, the line ablation rate decreases with time.

Ablation Behavior of the Coated Samples at Different Times
To evaluate the protective performance of antioxidant films against erosion in highvelocity airflow environments, a flame spray apparatus was employed to assess their ablation resistance. Figure 9a illustrates the filmʹs ablation process at a central temperature of 1000 °C and the thickness change of the ablation pit is calculated. Figure 9b presents the linear ablation rates of the film samples at different time intervals, revealing rates of 1.42 µm/min, 1.41 µm/min, and 1.04 µm/min at 4 min, 10 min, and 15 min, respectively. These results indicate a progressive reduction in film thickness and ablation rate as the duration of ablation increases. At the same time, the line ablation rate decreases with time. Figure 10 displays the XRD spectrum of the ablated film samples, demonstrating the presence of residual oxides primarily composed of B2O3, TiO2, and SiO2 on the surface. The oxide (B2O3 and SiO2) appears on the sample surface according to XPS (Figure 11), although their content is relatively less than annealing completed. The Si-O bond content was 9.43% (Figure 11c), lower than 41.66% (Figure 6c). SEM images (Figure 12(a,a1)) reveal the filmʹs intact morphology without visible voids or cracks after 4 min of ablation. However, after 10 min of ablation, surface melting occurs, exposing TiB2 and TiO2 particles ( Figure 12(b1,b2)). The low melting point of B2O3 (450 °C) leads to the volatilization of the surface oxides under the high-temperature conditions of 1000 °C. After 30 min of ablation, distinct ablation pits appear on the surface, accompanied by a decrease in the proportion of B and Si as observed through EDS analysis (Figure 12(a1,b1,c1)). Despite the presence of surface voids, cross-sectional analysis (Figure12c) confirms the filmʹs adherence to the alumina substrate without detachment or oxidation fractures. This integrity is maintained even after 4 min ( Figure 13a) and 10 min (Figure 13b) of ablation. After 30 min of ablation, a noticeable thinning of the cross-section (Figure 13c) is observed. During the ablation process, the possible oxidation reactions were shown as follows: SiO2·B2O3(l)→SiO2(g) + B2O3(g)    (Figure 11), although their content is relatively less than annealing completed. The Si-O bond content was 9.43% (Figure 11c), lower than 41.66% (Figure 6c). SEM images (Figure 12(a,a1)) reveal the film's intact morphology without visible voids or cracks after 4 min of ablation. However, after 10 min of ablation, surface melting occurs, exposing TiB 2 and TiO 2 particles (Figure 12(b1,b2)). The low melting point of B 2 O 3 (450 • C) leads to the volatilization of the surface oxides under the high-temperature conditions of 1000 • C. After 30 min of ablation, distinct ablation pits appear on the surface, accompanied by a decrease in the proportion of B and Si as observed through EDS analysis (Figure 12(a1,b1,c1)). Despite the presence of surface voids, cross-sectional analysis (Figure 12c) confirms the film's adherence to the alumina substrate without detachment or oxidation fractures. This integrity is maintained even after 4 min (Figure 13a) and 10 min (Figure 13b) of ablation. After 30 min of ablation, a noticeable thinning of the cross-section (Figure 13c) is observed. During the ablation process, the possible oxidation reactions were shown as follows:

Thin-Film Temperature Sensor Oxidation Resistance Ablation Test
By fabricating the thin-film coating directly on the alumina substrate, the oxidation and ablation exploration of the thin-film coating was completed. Finally, we used the electrical resistance characteristics of thin-film sensors to characterize the performance of ablation process coating. The thin-film coating was applied to the sensitive layer as an antioxidation layer forming a double-layer structure [16]. Figure 14a illustrates the optical image of the sensor prior to ablation, displaying a predominantly light yellow surface. After 30 min of ablation, the surface exhibited a deep blue color without any signs of cracking or delamination. Analysis of the film sensorʹs resistance variation demonstrated its impressive antioxidation performance even at the elevated temperature of 1000 °C ( Figure  14b). Upon undergoing pyrolysis [15] at 1000 °C, the film sensor experienced a decrease in resistance, ultimately reaching a stable state with a rate of change of 1.88% over 25 min (Figure 14c). After the ablation process is completed, the resistance of the thin-film sensor can be restored to its initial value. The cross-sectional image after 30 min ablation of the sensor is shown in Figure 15a. The antioxidant layer remains firmly bonded to the sensitive layer without any cracks, despite the destruction of the protective glass layer formed on the surface of the film. In addition, the EDS analysis of the sensor cross-section ( Figure   Figure 12. Surface SEM micrographs of different times: 4 min (a-a2); 10 min (b-b2); 30 min (c-c2); EDS analysis (a2,b2,c2) of spot 1, 2 (a1,b1,c1).

Thin-Film Temperature Sensor Oxidation Resistance Ablation Test
By fabricating the thin-film coating directly on the alumina substrate, the oxidation and ablation exploration of the thin-film coating was completed. Finally, we used the electrical resistance characteristics of thin-film sensors to characterize the performance of ablation process coating. The thin-film coating was applied to the sensitive layer as an antioxidation layer forming a double-layer structure [16]. Figure 14a illustrates the optical image of the sensor prior to ablation, displaying a predominantly light yellow surface. After 30 min of ablation, the surface exhibited a deep blue color without any signs of cracking or delamination. Analysis of the film sensorʹs resistance variation demonstrated its impressive antioxidation performance even at the elevated temperature of 1000 °C ( Figure  14b). Upon undergoing pyrolysis [15] at 1000 °C, the film sensor experienced a decrease in resistance, ultimately reaching a stable state with a rate of change of 1.88% over 25 min (Figure 14c). After the ablation process is completed, the resistance of the thin-film sensor can be restored to its initial value. The cross-sectional image after 30 min ablation of the sensor is shown in Figure 15a. The antioxidant layer remains firmly bonded to the sensitive layer without any cracks, despite the destruction of the protective glass layer formed on the surface of the film. In addition, the EDS analysis of the sensor cross-section (Figure

Thin-Film Temperature Sensor Oxidation Resistance Ablation Test
By fabricating the thin-film coating directly on the alumina substrate, the oxidation and ablation exploration of the thin-film coating was completed. Finally, we used the electrical resistance characteristics of thin-film sensors to characterize the performance of ablation process coating. The thin-film coating was applied to the sensitive layer as an antioxidation layer forming a double-layer structure [16]. Figure 14a illustrates the optical image of the sensor prior to ablation, displaying a predominantly light yellow surface. After 30 min of ablation, the surface exhibited a deep blue color without any signs of cracking or delamination. Analysis of the film sensor's resistance variation demonstrated its impressive antioxidation performance even at the elevated temperature of 1000 • C (Figure 14b). Upon undergoing pyrolysis [15] at 1000 • C, the film sensor experienced a decrease in resistance, ultimately reaching a stable state with a rate of change of 1.88% over 25 min (Figure 14c). After the ablation process is completed, the resistance of the thin-film sensor can be restored to its initial value. The cross-sectional image after 30 min ablation of the sensor is shown in Figure 15a. The antioxidant layer remains firmly bonded to the sensitive layer without any cracks, despite the destruction of the protective glass layer formed on the surface of the film. In addition, the EDS analysis of the sensor cross-section ( Figure 15b) shows a decrease in Si content above the antioxidant layer and the presence of a large amount of Si elements in the middle. Meanwhile, the Ti elements in the cross-section are uniformly distributed. During the ablation process, the vaporization of B 2 O 3 decreased the surface temperature. Since SiO 2 loses its stability above 2300 • C due to rapid evaporation [36,37], at 1000 • C, high viscosity SiO 2 flows with the ablation gas stream and it is difficult to evaporate, and liquid SiO 2 forms a dense layer on the ablation surface. By introducing Ti (TiB 2 ), the film did not exfoliate significantly and the impact resistance of the film was enhanced. 15b) shows a decrease in Si content above the antioxidant layer and the presence of a large amount of Si elements in the middle. Meanwhile, the Ti elements in the cross-section are uniformly distributed. During the ablation process, the vaporization of B2O3 decreased the surface temperature. Since SiO2 loses its stability above 2300 °C due to rapid evaporation [36,37], at 1000 °C, high viscosity SiO2 flows with the ablation gas stream and it is difficult to evaporate, and liquid SiO2 forms a dense layer on the ablation surface. By introducing Ti (TiB2), the film did not exfoliate significantly and the impact resistance of the film was enhanced.  To gain a comprehensive understanding of the oxidation and ablation resistance mechanism of the PDC/TiB 2 /B composite film, Figure 16 illustrates a simplified schematic. The thin-film coating is applied onto a sensitive layer that undergoes rapid oxidation and converts to Si, Ti, and B oxides at elevated temperatures. A highly viscous and fluid SiO 2 -B 2 O 3 glass layer is swiftly formed, effectively blocking the infiltration of oxygen. During the ablation process, the film surface temperature rises rapidly, leading to the volatilization of B 2 O 3 , which results in the formation of ablation products, such as SiO 2 and borosilicate, which are carried by the ablation airflow within and around the coating. While SiO 2 exhibits low evaporation at 1000 • C, it remains deposited on the film surface, forming a high-viscosity SiO 2 layer. This layer restricts the ingress of oxidizing gases into the film coating, thereby preventing further oxidation. To gain a comprehensive understanding of the oxidation and ablation resistance mechanism of the PDC/TiB2/B composite film, Figure 16 illustrates a simplified schematic. The thin-film coating is applied onto a sensitive layer that undergoes rapid oxidation and converts to Si, Ti, and B oxides at elevated temperatures. A highly viscous and fluid SiO2-B2O3 glass layer is swiftly formed, effectively blocking the infiltration of oxygen. During the ablation process, the film surface temperature rises rapidly, leading to the volatilization of B2O3, which results in the formation of ablation products, such as SiO2 and borosilicate, which are carried by the ablation airflow within and around the coating. While SiO2 exhibits low evaporation at 1000 °C, it remains deposited on the film surface, forming a high-viscosity SiO2 layer. This layer restricts the ingress of oxidizing gases into the film coating, thereby preventing further oxidation.

Conclusions
This study focuses on investigating the oxidation and ablation behavior of PDC/TiB2/B composites, which has an enhancing effect on the performance of PDC thinfilm sensors. Through high-temperature heat treatment, a dense SiO2-B2O3 oxide layer forms on the surface of the thin-film coating, effectively preventing further oxidation of the sensitive layer by oxygen. After oxidation in air at 800 °C for 10 h, the sample experiences only a 0.2% mass loss. Moreover, the generated oxide layer is also critical to the improvement of the ablation resistance. During high-temperature ablation at 1000 °C, the B2O3 on the coatingʹs surface absorbs and dissipates heat, leading to its significant volati-  To gain a comprehensive understanding of the oxidation and ablation resistance mechanism of the PDC/TiB2/B composite film, Figure 16 illustrates a simplified schematic. The thin-film coating is applied onto a sensitive layer that undergoes rapid oxidation and converts to Si, Ti, and B oxides at elevated temperatures. A highly viscous and fluid SiO2-B2O3 glass layer is swiftly formed, effectively blocking the infiltration of oxygen. During the ablation process, the film surface temperature rises rapidly, leading to the volatilization of B2O3, which results in the formation of ablation products, such as SiO2 and borosilicate, which are carried by the ablation airflow within and around the coating. While SiO2 exhibits low evaporation at 1000 °C, it remains deposited on the film surface, forming a high-viscosity SiO2 layer. This layer restricts the ingress of oxidizing gases into the film coating, thereby preventing further oxidation.

Conclusions
This study focuses on investigating the oxidation and ablation behavior of PDC/TiB2/B composites, which has an enhancing effect on the performance of PDC thinfilm sensors. Through high-temperature heat treatment, a dense SiO2-B2O3 oxide layer forms on the surface of the thin-film coating, effectively preventing further oxidation of the sensitive layer by oxygen. After oxidation in air at 800 °C for 10 h, the sample experiences only a 0.2% mass loss. Moreover, the generated oxide layer is also critical to the improvement of the ablation resistance. During high-temperature ablation at 1000 °C, the B2O3 on the coatingʹs surface absorbs and dissipates heat, leading to its significant volatilization. Simultaneously, the SiCN ceramic absorbs heat and undergoes further oxidation,

Conclusions
This study focuses on investigating the oxidation and ablation behavior of PDC/TiB 2 /B composites, which has an enhancing effect on the performance of PDC thin-film sensors. Through high-temperature heat treatment, a dense SiO 2 -B 2 O 3 oxide layer forms on the surface of the thin-film coating, effectively preventing further oxidation of the sensitive layer by oxygen. After oxidation in air at 800 • C for 10 h, the sample experiences only a 0.2% mass loss. Moreover, the generated oxide layer is also critical to the improvement of the ablation resistance. During high-temperature ablation at 1000 • C, the B 2 O 3 on the coating's surface absorbs and dissipates heat, leading to its significant volatilization. Simultaneously, the SiCN ceramic absorbs heat and undergoes further oxidation, resulting in the formation of new SiO 2 . This process replenishes the vaporized B 2 O 3 and contributes to the enhanced ablation resistance of the coating. After being exposed to the butane flame for 15 min, the coating demonstrates remarkable resistance to ablation, with a linear ablation rate of 1.04 µm/min. And the thin-film sensor exhibits an impressive resistance change rate of 0.0752%/min at 1000 • C. Consequently, the particle-filled PDC composite film coating has a key role in improving the oxidation and ablation performance of thin-film sensors. This work also provides insights and guidance for the design and development of thin-film coatings in extreme environments with high application potential.